1. Field of the Invention
Breathing apparatus of the type wherein the user rebreathes a portion of his previously exhaled breath.
2. Description of the Prior Art
Attempts have been made in the past to conserve the amount of breathing gas consumed from a breathing apparatus by allowing collection and subsequent rebreathing of the initial portion of exhaled gas.
This initial portion is low in carbon dioxide content and is referred to as the dead space or dead volume. Such breathing apparatus finds widespread use in the fields of diving, space, rescue work and in general, instances where the user must be supplied with breathable gas while in a hostile ambient environment.
Previous systems relied on a technique for capturing a fixed volume of exhaled gas within a distensible bag, the fixed volume being determined by maximum bag distension. The entire volume of gas captured within this bag was then made available for the next inhalation. Such systems increased the duration of the breathable gas supplied; however, in actuality, the dead volume is not a constant value nor is it a constant ratio with respect to inspiratory tidal volume. Accordingly, depending upon the fixed volume chosen, the apparatus can be less than efficient or in some instances, can be quite hazardous due to excess amounts of CO.sub.2 rebreathed.